wf_unregister_client() increments the client count when a client
unregisters. That is obviously incorrect. Decrement that client count
instead.

Fixes: 75722d39 ("[PATCH] ppc64: Thermal control for SMU based machines")
Signed-off-by: NPaul Bolle <pebolle@tiscali.nl>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>